Liquid crystal display (LCD) in general, and twisted-nematic (TN) LCDs in particular, exhibit a strong effect of viewing direction on their optical properties. FIG. 1 shows two light rays traveling through an LCD cell 10 at normal 11 and oblique 12 directions. Since the properties of the linear polarizers on the exterior of the cell and the length of the optical path through the LCD both change with incidence angle, the light intensity and the contrast of the LCD are strongly affected by the direction under which the observer views the display. Thus, the traditional problem of narrow viewing angle arises.
Some have attempted to solve this problem by increasing the contrast of the LCD. This has been done by adjusting the driving voltage to the liquid crystal (LC) cell. Typical TN LCDs are driven with AC-voltage using square wave signals of constant amplitude. When the display is driven with standard multiplexing signals the modulations in the liquid crystal fluid become pronounced. Stimulated by the selected pulse, the optical axis of the liquid crystal rotates towards the direction of the electrical field at the beginning of the frame period and it relaxes toward the initial state during the rest of the frame. This modulation of transmitted or reflected light is called the frame response. Though the human eye averages these modulations if their depth and frequency are within certain limits, frame response severely reduces the contrast of highly multiplexed LCDs by brightening up the `dark` state of positive contrast LCDs. Increasing the driving voltage improves the contrast of the LCD, but decrease the effective viewing angle.
In the ideal situation, the display of an LCD would appear the same no matter what the position of the observer is relative to the LCD. This concept of appearance over a wide range of positions is known as the `viewing cone`. FIG. 2 shows a schematic representation of the ideal viewing cone, where an LCD 20 situated in the center of the cone 22 appears the same to an observer 24 positioned anywhere on the hemisphere. Practical considerations such as those enumerated above severely distort the viewing cone so that it is often very narrow. Present day LCDs are designed to have various shaped viewing cones, depending on the location of the display relative to the user. Increasing the contrast ratio, as described above, results in narrowing the viewing cone. Thus, traditional attempts to improve one of these properties have degraded the other. It would be desirable if one could create an LCD that has high contrast and a wide viewing cone.